1. Field of the Invention
The present invention relates to a method of preparing a porous material incorporating ultrafine metal particles, and more particularly to a method of preparing a porous material incorporating ultrafine metal particles of diameter a few nanometers that can be used, for example, as a catalyst or a photonics material that uses a minute size effect, and to the porous material incorporating ultrafine metal particles prepared using the method.
2. Background of the Invention
Moreover, it is expected that materials in which ultrafine metal particles are dispersed through a transparent matrix should be usable as photonics materials that make use of the highly nonlinear third order optical properties characteristic of ultrafine particles. The method generally used to prepare such materials is to mix a metal salt into a solution of an alkoxide of silicon or the like, and precipitate out ultrafine particles during a matrix gelation process by heating and adding a reducing agent.
Conventionally, the method most commonly used to prepare a material in which ultrafine metal particles are incorporated in a porous material is to first prepare a porous body having a honeycomb shape or the like, then make this porous body come into contact with a solution containing metal ions, and then carry out heat treatment/reduction (reference: Koji Onishi, Shokubaixe2x80x94Sono Himitsu o Saguruxe2x80x94(xe2x80x98Probing the Secrets of Catalystsxe2x80x99), Dainippon-tosho, 1987). However, with this method, the porous body must have sufficient strength so as not to be damaged by the metal-introducing process, and hence the surface area of the porous body per unit weight cannot be made very high, and thus the amount of metal that can be introduced is limited. As a result, the catalyst quantity per unit weight of the material cannot be made very high. Moreover, because the fine metal particles are produced through heat treatment/reduction, sintering occurs during the heat treatment, resulting in relatively large fine particles of diameter of the order of several microns.
Moving on, a method commonly used to disperse ultrafine metal particles of diameter a few nm to a few tens of nm through a support is to prepare the ultrafine metal particles in a gel (reference: ed. Ueno, Mizukami and Sodezawa, Kinzoku-Arukokishido o Mochiiru Shokubai Chosei (xe2x80x98Preparation of Catalysts using Metal Alkoxidesxe2x80x99), ICP, 1993). In this method, a salt or complex of the metal that one wishes to incorporate is added to a solution of a metal alkoxide that acts as a precursor of the support, and then hydrolysis and gelation are carried out, followed by drying. Ultrafine metal particles are then precipitated out into the support by heating to a few hundred, or else instead of heating, the ultrafine particles are produced at room temperature by adding a reducing agent. By using this method, ultrafine metal particles can be supported at any desired proportion in a fine support network, but because the precipitation of the ultrafine metal particles occurs at the heat treatment/reduction stage after the gel has been dried, there is a problem that it is difficult to control the size of the particles; moreover, in the case of heat treatment, sintering of the particles occurs, and hence there is a problem that relatively large ultrafine particles are produced.
Ultrafine particles of a metal such as gold of diameter a few nm cannot exist as is in a solution, the air or the like, but rather agglomeration occurs. However, by adsorbing protective groups such as thiol groups onto the surfaces of the ultrafine metal particles, such agglomeration can be inhibited. Specifically, it is known that by reducing metal ions in chloroauric acid or the like in the presence of an alkylthiol or the like, the metal can be made to stably exist in the form of ultrafine particles of diameter a few nm. Moreover, by controlling the preparation conditions, it is possible to control the size of the ultrafine metal particles produced.
However, art for dispersing such ultrafine metal particles through a porous body in a form in which the surface-protecting groups such as thiol groups have been removed so that usage as a catalyst is possible, and art for dispersing such ultrafine metal particles through a transparent inorganic substance so that application to photonics elements is possible, has not yet been developed.
With the foregoing in view, the present inventors carried out assiduous studies with a goal of developing art for introducing ultrafine metal particles of diameter down to a few nm that have been pre-prepared with size control carried out into a porous inorganic material in any desired proportion without changing the size of the ultrafine particles. As a result, the present inventors discovered that this goal can be attained by immersing a wet gel in a solution of surface-protected ultrafine metal particles, drying the resulting ultrafine metal particle/wet gel composite, and heating the resulting dried body to remove the surface-protecting molecules. As a result, the present inventors arrived at the present invention.
A porous material incorporating ultrafine metal particles and a method of preparing the same are provided. The method of preparing the porous material incorporating ultrafine metal particles comprises the following steps: (1) preparing surface-protected ultrafine metal particles by reducing metal ions in the presence of molecules such as dodecanethiol molecules; (2) immersing a wet gel in a solution of the ultrafine metal particles, thus forming an ultrafine metal particle/wet gel composite in which the ultrafine metal particles are incorporated in the wet gel; and (3) drying the ultrafine metal particle/wet gel composite to form a porous body. Moreover, the surface-protecting molecules are subsequently removed by heating the porous body.
It is thus an object of the present invention to manufacture and provide an ultrafine metal particle/aerogel composite by preparing ultrafine metal particles using a liquid phase method, incorporating the ultrafine metal particles in a gel to produce an ultrafine metal particle/wet gel composite, and then drying the ultrafine metal particle/wet gel composite.
To solve the above problems, the present invention is constituted from the following technical means.
(1) A method of preparing a porous material incorporating ultrafine metal particles, comprising the steps of:
preparing surface-protected ultrafine metal particles by reducing metal ions in the presence of thiol groups or other surface-protecting groups;
immersing a wet gel in a solution of the ultrafine metal particles, thus forming an ultrafine metal particle/wet gel composite in which the ultrafine metal particles are incorporated in the wet gel; and
drying the ultrafine metal particle/wet gel composite.
(2) The method of preparing a porous material incorporating ultrafine metal particles described in (1) above, wherein the metal is one or more selected from the group consisting of gold, silver, palladium and other noble metal, and iron, cobalt and other transition metal.
(3) The method of preparing a porous material incorporating ultrafine metal particles described in (1) above, wherein the ultrafine metal particles have a particle diameter of 1 to 20 nm.
(4) The method of preparing a porous material incorporating ultrafine metal particles described in (1) above, wherein the solvent in which the ultrafine metal particles are dissolved is toluene, hexane, and/or tetrahydrofuran.
(5) The method of preparing a porous material incorporating ultrafine metal particles described in (1) above, wherein the wet gel used is a silica wet gel or an alumina wet gel.
(6) The method of preparing a porous material incorporating ultrafine metal particles described in (1) above, wherein the ultrafine metal particle/wet gel composite is dried by natural drying or supercritical drying.
(7) A method of preparing a porous material incorporating ultrafine metal particles, comprising the step of heating the porous material prepared by the method described in (1) above to remove the surface-protecting molecules.
(8) A porous material supporting ultrafine metal particles, comprising an ultrafine metal particle/aerogel composite, prepared by the method as described in any of (1) to (7) above.